Indian summer is a period of unseasonably warm, dry weather that sometimes occurs in autumn in the Northern Hemisphere. The US National Weather Service defines this as weather conditions that are sunny and clear with above normal temperatures, occurring late-September to mid-November.[1] It is usually described as occurring after a killing frost.[2]

The Old Farmer's Almanac has additional criteria:

As well as being warm, the atmosphere during Indian summer is hazy or smoky, there is no wind, the barometer is standing high, and the nights are clear and chilly. A moving, cool, shallow polar air mass is converting into a deep, warm, stagnant anticyclone (high pressure) system, which has the effect of causing the haze and large swing in temperature between day and night. The time of occurrence is important: The warm days must follow a spell of cold weather or a good hard frost. The conditions described above must occur between St. Martin's Day (November 11) and November 20. For over 200 years, The Old Farmer's Almanac has adhered to the saying, "If All Saints' (November 1) brings out winter, St. Martin's brings out Indian summer."

Shakespeare used the expression "All-Hallowe'en Summer" in Henry IV: ("Farewell, thou latter spring; farewell, All-hallown summer!” Henry IV part 1, act I, scene 2). "All Hallows'" or All Saints' Day, falls on November 1, so a sunny period at this time of year could be referred to in this way.[Speculation]

Late-19th century Boston lexicographer Albert Matthews made an exhaustive search of early American literature in an attempt to discover who coined the expression. The earliest reference he found dated from 1778, but from the context it was clearly already in widespread use.[3]

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A famous use of the phrase in American literature is the title of Van Wyck BrooksNew England: Indian Summer (1940), chosen to suggest inconsistency, infertility, and depleted capabilities, a period of seemingly robust strength that is only an imitation of an earlier season of actual strength.[4]

One of the many beautiful trees in late bloom during Indian summer in New Hampshire.

In British English the term "Indian summer" is today used loosely for a period of unseasonable warmth and sunshine in late September, October, or November. In the UK, observers knew of the American usage from the mid-19th century onwards, and The Indian Summer of a Forsyte is the metaphorical title of the 1918 second volume of The Forsyte Saga by John Galsworthy. 20th century climatologists including Gordon Manley and Hubert Lamb used it only when referring to the American phenomenon, and the expression did not gain wide currency in Britain until the 1950s.

The term may also refer to the weather patterns in the Indian Ocean, where ships' hulls were marked "I.S." to indicate the level at which they should be loaded during that season.[1]

In former times such a period was associated with the October feast days of St. Martin and Saint Luke.

In Bulgaria it is known as Gypsy summer [Cigansko lyato, циганско лято], less often as Poorman's summer [Siromashko lyato, сиромашко лято]. While in Ukraine it is known as Babine lito (Grandmother's summer) .